I have very very small gravel smaller than pea gravel, but its the same colour as sand and looks like sand from a distance, the corries dig in that the same as when they had sand, and their barbels are still long so no wearing down of them.
Changed back as found sand a bit hard to keep clean and looking the way I wanted it to.
I think sand is the best for corries, and a lot of people swear by it, this is my compromise.